Synthesis of advanced scintillators via vapor deposition techniques

ABSTRACT

Transparent optical ceramic coating materials have been fabricated from europium-doped lutetium oxide (Lu 2 O 3 :Eu) using physical vapor deposition and chemical vapor deposition techniques. The non-pixilated film coatings have columnar microcrystalline structure and excellent properties for use as radiological scintillators, namely very high density, high effective atomic number, and light output and emission wavelength suitable for use with silicon-based detectors having a very high quantum efficiency. The materials can be used in a multitude of high speed and high resolution imaging applications, including x-ray imaging in medicine.

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 61/149,880 filed Feb. 4, 2009 andentitled “SYNTHESIS OF ADVANCED SCINTILLATORS VIA VAPOR DEPOSITIONTECHNIQUES” which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The research leading to this invention was carried out with U.S.Government support provided under Grant No. 5R21EB005037 from theNational Institutes of Health. The U.S. Government has certain rights inthe invention.

BACKGROUND

Rare earth oxides are used in the x-ray detector industry due to theirstability, high density, and high atomic number. However, they havegenerally been limited to small area detectors due to manufacturinglimitations. The present industry standard scintillator for x-raydetection is cesium iodide doped with tantalum (CsI:Tl). In terms ofoptical and scintillation properties, CsI:Tl has good transparency, adensity of 4.51 g/cc, and emits ˜60,000 photons per MeV of incidentx-rays [1]. Lutetium Oxide doped with Europium Oxide (Lu₂O₃:Eu) has beenstudied as an alternative to CsI:Tl, because its high density and highatomic number make it an ideal scintillator. Lu₂O₃:Eu has a highlytransparent body-centered cubic (BCC) crystal structure, a density of9.4 g/cc, and it emits ˜30,000 photons per MeV [2].

Current manufacturing methods, such as sintering and hot pressing,produce a transparent 2-3 mm thick disc that must be ground and polishedto a thickness close to the desired thickness. In order to reduce lightscattering, the disc must then be pixelized as shown in FIG. 1 using ahighly labor intensive laser ablation process. The top surface is thenplaced onto a CCD camera using optical glue, and the back is ground off[1]. Use of Lu₂O₃:Eu scintillators in dentistry, which is one of thepotential applications for such a material, would require a ceramicdetector of approximately 200 microns thickness in order to absorb mostof the incoming x-rays, compared to 2 mm thickness for CsI. With currentfabrication technology, this is not commercially viable, due to therequired processing. There is a need for improved methods ofmanufacturing Lu₂O₃:Eu scintillator material that would be moreefficient, less labor intensive, and more suitable to produce large areascintillators than with currently available techniques.

SUMMARY OF THE INVENTION

The invention provides scintillator coating materials and films thatprovide superior radiological imaging. The materials are based onlutetium (e.g., Lu₂O₃, Lu₂SiO₅, etc.), which has a high atomic numberand is therefore highly efficient at capturing high energy photons, suchas x-rays. The lutetium is in the form of, for example, lutetium oxide(Lu₂O₃) doped with europium oxide (Eu₂O₃). The lutetium oxide andeuropium oxide form a solid solution having an oriented columnar graingrowth pattern. Lutetium serves to trap incident x-rays, whose energy istransferred to europium, causing an electron orbital shift in europiumthat results in the release of visible light photons. The columnargrowth structure eliminates the need for pixelation and provides highlyefficient light transmission out of the scintillator material. Theemitted visible light is in a wavelength range that can be imaged withhigh efficiency using solid state silicon devices such as CCDs.

One aspect of the invention is a method of preparing a radiologicalscintillator coating material by physical vapor deposition (PVD). Themethod includes the steps of providing a target and a substrate to becoated with the scintillator coating material and subjecting the targetto a physical vapor deposition process. The substrate is formed from acompressed powder of Lu₂O₃ doped with about 5-15 mol % Eu₂O₃. In someembodiments, the PVD method is plasma sputtering in a radio frequencymagnetron sputtering system using an argon plasma. As a result of thephysical deposition process, a scintillator coating comprising Lu₂O₃ andEu₂O₃ is deposited onto the substrate. In some embodiments, the methodfurther includes the step of annealing the scintillator coating by heattreatment at a temperature in the range of about 100 to 1400° C.

Another aspect of the invention is a method of preparing a radiologicalscintillator coating material by chemical vapor deposition (CVD). Themethod includes the steps of providing chemical reactants and asubstrate to be coated with the scintillator coating material andreacting the reactants in a CVD reactor. The reactants include LuCl₃,EuCl₃, CO₂, and H₂. As a result of the CVD process, a scintillatorcoating comprising Lu₂O₃ and Eu₂O₃ is deposited onto the substrate. Theratio of LuCl₃ and EuCl₃ is adjusted to provide a ratio of about 85-95mol % Lu₂O₃ and about 5-15% Eu₂O₃ in the scintillator coating. In someembodiments, the method further includes the step of annealing thescintillator coating by heat treatment at a temperature in the range ofabout 100 to 1400° C. In some embodiments, the LuCl₃ and EuCl₃ reactantsare generated in the reactor by reacting Cl₂ gas with Lu metal and Eumetal.

Yet another aspect of the invention is a non-pixilated radiologicalscintillator coating material including about 85 to 95 mol % Lu₂O₃ andabout 5 to 15 mol % Eu₂O₃. The material has a preferentially orientedcolumnar grain growth structure, absorbs electromagnetic radiationincluding x-rays, and in response emits visible light. The material canbe used in radiological imaging applications in medicine and dentistry.

Still another aspect of the invention is an x-ray imaging device. Thedevice includes a scintillator coating material made of about 85 to 95mol % Lu₂O₃ doped with about 5 to 15 mol % Eu₂O₃. The scintillatorcoating is deposited onto a substrate or a CCD imaging device.Optionally, the CCD is in turn mounted on a circuit board material suchas FR4. The device can be linked to a microprocessor, a memory unit, anda display unit, such as a computer, to display images formed by x-raysimpinging on the scintillator material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scanning electron micrograph of a prior art Lu₂O₃:Euscintillator material that has been pixelized by laser ablation.

FIG. 2 shows a diagram of a physical vapor deposition (PVD) system usinga radio frequency (RF) magnetron sputtering system.

FIG. 3 shows the effect of RF power on deposition rate of Lu₂O₃:Eu by aPVD process. Morphology of the material is shown above each condition byscanning electron microscopy (SEM) in cross section and top view.

FIG. 4 shows the results of x-ray diffraction analysis on Lu₂O₃:Eudeposited by a PVD process at room temperature for different RF powersettings (50, 75, and 100 watts).

FIG. 5A shows the results of x-ray diffraction analysis of Lu₂O₃:Eudeposited by PVD at 400° C. and treated subsequently at the indicatedtemperatures. FIGS. 5B and 5C show morphology by SEM of the 400° C.material as deposited.

FIG. 6 shows a diagram of cold wall and hot wall chemical vapordeposition (CVD) reactors that can be used to deposit Lu₂O₃:Eu accordingto the invention.

FIGS. 7A-7D show the results of Femlab modeling of CVD flow dynamics (7Aand 7C) and temperature profiles (7B and 7D) for two differentseparation distances between the crucible and substrate (1.25″ for 7Aand 7B, 3″ for 7C and 7D).

FIGS. 8A-8C show the morphology by SEM of CVD films of Lu₂O₃:Eudeposited at substrate temperature of 950° C. and CO₂ flow of 300 sccm(FIG. 8A), 1000° C. and CO₂ flow of 400 sccm (FIG. 8B), and 1050° C. andCO₂ flow of 500 sccm (FIG. 8C).

FIGS. 9A and 9B show the morphology by SEM of Lu₂O₃:Eu deposited by CVDwith stacked platelet columnar morphology in cross section (FIG. 9A) andsurface view (FIG. 9B). Growth was at 1000° C.

FIG. 10A shows the results of x-ray diffraction analysis of a stackedplatelet columnar coating of Lu₂O₃:Eu deposited by CVD. The diffractionpattern of single phase polycrystalline lutetium oxide powder is shownin FIG. 10B for comparison.

FIGS. 11A and 11B show SEM of highly faceted columnar Lu₂O₃:Eu depositedby CVD in cross section (FIG. 11A) and surface view (FIG. 11B).

FIG. 12 shows an x-ray diffraction plot of highly faceted columnarLu₂O₃:Eu deposited by CVD.

FIG. 13 shows a cathodoluminescence spectrum of highly faceted columnarLu₂O₃:Eu deposited by CVD.

FIGS. 14A and 14B show radioluminescence emission spectra forEu-activated Lu₂O₃ for a CVD film (FIG. 14A) and a hot-pressed ceramicfilm (FIG. 14B).

FIG. 15 shows radioluminescence emission spectra of [CVD???] depositedLu₂O₃:Eu films that were annealed at the indicated temperatures afterdeposition. Peak emission is a monotonically increasing function of thetemperature (highest value at 900° C. annealing temperature).

FIG. 16 shows the modulation transfer function (MTF) value of a Lu₂O₃:Eucoating deposited by CVD (upper curve) compared to the published valuesfor a pixelated ceramic (lower curve, from [5]).

FIGS. 17A and 17B show x-ray diffraction analysis and morphology by SEMof heat treated (900° C., 2 hours) Lu₂O₃:Eu deposited by PVD at theindicated RF power settings (75 watts for FIG. 17A, 50 watts for FIG.17B). The (100) peak is shown for the as deposited material forcomparison.

FIGS. 18A-18C show photoluminescence images (254 nm excitation) of PVDcoatings of Lu₂O₃:Eu made using RF power settings of 50 W (FIG. 18A), 75W (FIG. 18B), and 100 W (FIG. 18C). For each coating, the left half ofthe figure shows a coating as deposited, and the right half shows acoating that was heat treated at 900° C. for 2 hours.

FIG. 19 shows an x-ray image of an integrated circuit chip recordedusing radioluminescence from a Lu₂O₃:Eu coating according to theinvention [PVD or CVD?] glued to a CCD camera.

FIG. 20 is a schematic diagram illustrating an embodiment of an x-rayimaging device for dentistry featuring a Lu₂O₃:Eu scintillator filmaccording to the invention mounted on a CCD imaging chip.

DETAILED DESCRIPTION OF THE INVENTION

Novel Eu₂O₃-doped Lu₂O₃ scintillator materials and methods of makingthem are provided by the present invention. The new materials are madeby vapor deposition techniques, including physical vapor deposition(PVD) and chemical vapor deposition (CVD). The materials can be used ascoatings or films and are particularly well suited to serve asscintillators for radiological imaging devices, allowing the real timeacquisition of images in digital form. The invention also providesimaging devices that incorporate the scintillator materials.

The new materials utilize high energy photon capture by lutetium, whichhas a high atomic number and very high density. The lutetium is presentin the scintillator materials in the form of lutetium oxide (Lu₂O₃) thathas been doped with europium oxide (Eu₂O₃). The lutetium oxide andeuropium oxide form a solid solution whose morphology reveals anoriented columnar grain structure. The europium dopant converts capturedx-rays into emitted visible light photons.

The columnar grain structure of the scintillator materials eliminatesthe need for pixelation and provides highly efficient light transmissionout of the scintillator material, similar to transmission of light in anoptical fiber. By eliminating the need for pixelation, the materials ofthe invention avoid laborious and time-consuming steps in the productionof scintillator films, and make possible the production of much largerarea scintillator films than could be practically achieved using thepixelation and coating methods required for hot-pressed Lu₂O₃:Eumaterials, in order to minimize light scattering. The size or surfacearea that can be achieved will depend on and be limited by the size ofthe target in PVD and the size of the reactor in CVD. The emittedvisible light is in the 600 nm range, a range that can be imaged withhigh efficiency and recorded directly in digital form using CCDs.

The key to the production of the new scintillator materials is the useof vapor deposition techniques. The materials are made by either a PVDmethod or a CVD method. Further, in order to achieve high efficiencylight emission from the scintillator materials, a post-depositionthermal annealing step is performed, which cures defects in thematerial, allowing efficient energy transfer resulting in lightemission.

PVD methods that are suitable for synthesizing Lu₂O₃:Eu films withappropriate structure for use as a scintillator include, but are notlimited to, evaporative deposition, cathodic arc deposition, ionbombardment, electron beam bombardment, and sputtering. In evaporation,a material is heated in a vacuum to increase its vapor pressure,resulting in deposition of the vaporized material. Arc deposition uses ahigh power electrical arc to vaporize a target, resulting in depositionof vaporized material. Sputtering utilizes a plasma discharge to sputteraway atoms from a target material into a vapor, which are then depositedonto a substrate. Ion beam or electron bombardment use the respectivebeam to vaporize material from a target and deposit the vaporizedmaterial onto a substrate. A preferred PVD method is radio frequencymagnetron sputtering. Phase diagrams for films deposited by a PVDprocess such as sputtering are known from previous studies. From suchprevious studies it is understood that the columnar grain structure,which is preferred for the materials of the present invention, arepreferentially formed at high substrate temperatures.

Any CVD method that provides an appropriate grain morphology and Lu:Eustoichiometry can be used to manufacture the scintillator films of theinvention. CVD of films or coatings involves the chemical reaction ofgaseous reactants on or near the vicinity of a heated substrate surface.This atomistic deposition method can provide high purity materials withstructural control at atomic or nanometer scale level. Moreover, it canproduce single layer, multilayer, composite, nanostructured, andfunctionally graded coating materials with well controlled dimension andunique structure at low processing temperatures. Furthermore, one of theunique features of CVD compared with other deposition techniques is itsnon-line-of-sight deposition capability, which allows the coating ofcomplex shaped components.

In addition, CVD can be carried out employing hot or cold wall reactors.In hot wall CVD, the deposition chamber is heated, which in turn heatsthe gases through conduction and radiation. Though the hot wall reactorcan provide very precise temperature control, the interior of the hotwall reactor is also coated (heterogeneous nucleation) and can inducegas phase (homogeneous) nucleation, resulting in maintenance problemsand lower deposition efficiency. In addition, depletion of gaseousreactants also occurs along the reactor requiring complex systems forlarge substrates.

In a cold wall reactor only the substrate is heated, either inductivelyor resistively, and the wall of the reactor is cold. Most CVD reactionsare endothermic. Therefore, the deposition reaction will occur only onthe heated substrate, and negligible deposition occurs on the wall ofthe reactor. Although these reactors are more complex, they allowgreater control over the deposition process, enabling higher qualitycoatings. However, thermal convection, which occurs in a cold wallreactor, can create concentration gradients of the reactive species andcan sometimes result in non-uniform coatings. This can be overcome byperforming CVD cold wall deposition at a reduced pressure. Factors thatdetermine the heating method are the size and geometry of the substrateand whether it is conducting or non-conducting. Additionally, by usingcold wall CVD and thus avoiding homogeneous nucleation, higher growthrates can be achieved. This drastically reduces the deposition timerequired to achieve the scintillator coating thickness necessary toabsorb most of the incident radiation.

The analysis and optimization of CVD processes requires the applicationof thermodynamics, chemical kinetics, and mass transport phenomena. Anunderstanding of these parameters allows the user to control thestructure, stoichiometry, crystallinity and texture of films [6, 7]. Theeffects of temperature and supersaturation on growth morphology for aCVD process are known from previous studies. Preferred morphologies forthe present invention are platelets and epitaxial growth, whileamorphous deposits are to be avoided.

Whether made by PVD or CVD, a scintillator film of the invention isdeposited onto a substrate. Suitable substrate materials are preferablysmooth, mechanically rigid, largely transparent to x-rays, and eitherhighly reflective or highly transparent to the light emitted by thescintillator. The substrate material should be able to withstand theconditions used for PVD or CVD without significant degradation thatwould impact film structure or integrity. Examples of suitable substratematerials include, but are not limited to, graphite, quartz, andfiberoptic plate material. The substrate is required for the depositionprocess, but it can be removed, for example, by mechanically grinding itaway, after the film is attached to another structure (e.g., a CCD) atits surface facing away from the substrate.

Scintillator materials containing Lu₂O₃:Eu can be incorporated into avariety of devices, particularly optical devices designed to convertx-rays into visible light for quantification or imaging of an x-raysource, or for imaging of an object that scatters or absorbs x-rays. Anexample of an embodiment suitable for dentistry is shown in FIG. 20.X-rays impinge on the device from the top of the figure (arrows). Thescintillator material in the form of film 10 is adhered to a CCD device,shown as layer 20. The thickness of the scintillator will depend on theapplication; however, the scintillator film should be sufficiently thickto absorb most of the incident x-rays to be imaged or quantified. Forexample, scintillator layer 10 can be in the range from about 50-500 μmin thickness, preferably about 200 μm thick. The thickness of CCD layer20 can be, for example, about 350 μm. The CCD layer can be supported bycircuit substrate 30. The circuit substrate can be any suitablematerial, but is preferably a material such as FR4, a glass fiber-epoxyresin material used in printed circuit boards, which is electricallyinsulating and rigid. Optionally, one or both faces of the device areencased in a layer of housing material 40, such as a plastic material orother material that is transparent to x-rays, at least on the sidefacing the incoming x-rays. Cable 50 connects the CCD layer to a devicesuch as a computer for input, analysis, display, and storage of signalsfrom the CCD, such as images. Devices containing the Lu₂O₃:Euscintillator material can be used for any purpose related to detectionof x-rays by scintillation. For example, such uses include recordingdental x-rays; recording any type of medical x-rays such as inmammography, chest x-rays, or diagnostic x-rays; and recording images orperforming analysis of any object that scatters or absorbs x-rays,including metals, microelectronics components, and nanomaterials.

The invention contemplates a method of preparing a radiologicalscintillator coating material by a vapor deposition technique. To makethe coating material, a layer of lutetium and europium oxides isdeposited onto a substrate by a physical or chemical vapor depositiontechnique. The ratio of lutetium to europium is selected such that thedeposited layer provides effective scintillation in response to incidentradiation. The incident radiation is a high energy, short wavelengthradiation, such as x-rays. Preferably, the deposited layer issubsequently annealed by heat treatment at a temperature in the range ofabout 100 to 1400° C., so as to improve the emission characteristics.The invention also contemplates a device for x-ray imaging. Such adevice includes the scintillator coating material just described and asemiconductor imaging device. The device can have a configuration suchas that shown in FIG. 20, or another configuration suitable for aparticular imaging application.

EXAMPLES Example 1 Physical Vapor Deposition of Lu₂O₃:Eu Films

Films of Lu₂O₃:Eu³⁺ were successfully deposited using physical vapordeposition (PVD) carried out in a radio frequency (RF) magnetronsputtering device (see FIG. 2). The setup used a 2 inch diameter targetangled at 45 degrees with respect to the substrate. The target was madeby hot pressing Lu₂O₃ powder doped with 5 mol % Eu₂O₃ at 1700° C. usinga graphite uniaxial hot press. A thin 2 inch diameter graphite disc wasused as the substrate and it was rotated at approximately 20 rpm toincrease uniformity. The RF power source was an Advanced Energy RFX600capable of producing 600 Watts. Coatings were deposited at 50, 75 and100 watts and examined. It was determined that 100 watts was the maximumuseable power level, above which charging and target damage occurred.The coatings were examined using a Bruker D8 Focus X-ray diffraction(XRD) unit using Cu-Kα radiation to determine orientation, and a Zeissfield emission scanning electron microscope (SEM) to examine themicrostructure.

Microstructural analysis of the top surface and the fractured crosssections, as shown in FIG. 3, revealed that growth morphology dependedon input power. The surface images showed a clear transition from whatappears to be cellular growth to plate growth. At 50 W and 100 W thecolumnar growth appeared to be of uniform width and perpendicular to thesurface, whereas at 75 W, the columnar growth became radial. Thediameter of the columnar growth is not clear from the fractured crosssection. However, top surface images shown in FIG. 3 clearly show largerboundaries, indicative of columnar grain growth. Grain diametermeasurements indicated a trend of decreasing columnar diameter withincreasing power (or deposition rate) as shown in Table 1. The columnargrowth was determined to be (100) textured for low input power and (111)textured for high input power as determined from the XRD pattern (FIG.4). It is noteworthy that the intensities of the (100) and (111) peakswere low, indicating that crystallinity/orientation was not significant.Low intensity diffraction peaks from other planes further suggest thatnot all growths were perpendicular and growth was slightlypolycrystalline. This is most likely a result of slow kinetics, becausethe low thermal energy did not enable the newly formed grains to growepitaxially. Furthermore, the XRD patterns were increasingly shiftedtowards a larger unit cell with increasing power, which is typicallyattributed to growth stresses. The (100) orientation is a lower energygrowth direction, and with sufficient stresses it can shift towards(111) growth.

TABLE I X-ray diffraction analysis compared with SEM grain sizemeasurements Power Measured Grain (Watts) Size (nm) Lattice DistortionVolume Distortion 50 415 1.0% 3.0% 75 290 1.2% 3.6% 100 247 2.1% 6.3%

In a PVD sputtering system, the plasma intensity is dependent on thepower applied, which also affects the sputtering rate. The plasma itselfcan attain high temperatures and can provide some thermal energy to thecoating, and the substrate can reach temperatures up to 100° C. However,the plasma provides a relatively large amount of thermal energy to avery thin layer, notably the deposition layer. This is believed to bethe reason for the drastic change in coating morphology observed at 75W. At this power there is a balance between deposition rate and thermalenergy provided by the plasma that enables better crystallization. At 50W the low intensity plasma provides low thermal energy and, despitereduced deposition rates, is not adequate for crystalline growth. At 100W, despite increased plasma thermal energy, the atoms did not havesufficient time to rearrange because of the higher density of incomingatoms.

Example 2 PVD Films Deposited on Heated Substrates

Deposition by PVD at a substrate temperature of 400° C. resulted in acoating that exhibits a significantly higher degree of orientation thanthat obtained at room temperature. FIG. 5A shows a plot of the XRD datafor films deposited at increasing substrate temperature. The peaksremain slightly shifted toward higher unit cell dimensions, indicatingsome residual stress and suggesting the need for even greater thermalenergy, perhaps by heating to 700-900° C. The micrograph of thesefracture cross-sections (FIG. 5B) reveals a columnar structure, and thecorresponding surface micrograph (FIG. 5C) contains randomly orientedpyramidal shapes similar to the 75 W coating in FIG. 3, suggesting thatheated substrates accommodate higher material fluxes than they wouldotherwise tolerate at room temperature.

Example 3 Effect of Heat Treatment on Lu₂O₃:Eu Films Deposited by PVD

To study the effects of annealing by heat treatment, coatings werepost-treated in a tungsten furnace at 900° C. in an argon atmosphere for2 hours. The samples from Example 1 were heat treated to increasecrystallinity and observe changes in morphology. As seen in FIG. 17, the(100) peak reverted back to the theoretical position after heattreatment, indicating stress relief. However, associated with therestored unit cell is a subsequent volume change resulting in reducedthickness and cracking (FIG. 17). In the 100 W case, the volumedistortion led to loss of adhesion, making further analysis on the heattreated sample almost impossible. One can observe in FIG. 17 that themorphology of the coating remained identical to the as-depositedcoating, indicating stability of the coating. A small increase in theintensity of the (100) peak was observed, indicating slight grain growthor increase in crystallinity. In FIG. 18C it can be seen that the edgeof the 100 W sample remained adherent, which can be attributed to thenon-uniformity of deposition conditions. These films were deposited on agraphite substrate, and adhesion might be different with anothersubstrate material. In magnetron sputtering, a ring source is created,which in the present case was angled at approximately 45 degrees to arotating substrate. The angling and rotation was used to improvethickness uniformity, but resulted in non-uniform plasma heating anddeposition angles, which are critical growth factors. Furthermore, thekinetic energy of the ejected material plays a crucial role in thecoating properties and is a function of the travel distance and totalpressure. Therefore, the center of the substrate was exposed torelatively constant deposition conditions, while the outer edges variedsignificantly every half rotation. The XRD pattern of the outer edge wasthat of a partially polycrystalline coating.

One of the indicators of the extent of crystallization in ascintillating material is the emission intensity and spectrum. Theemission spectrum of the as deposited and heat treated samples weremeasured using cathodoluminescence. The emission intensity for the asdeposited sample was found to be too low to be detected, while theheat-treated samples had a standard emission spectrum. Ultraviolet lightat 254 nm also induces emission due to the charge transfer band atapproximately 250 nm in the host material, as seen in FIG. 18A-18C. Thelack of emission by the as deposited samples can be attributed to eitherlow crystallinity or a large number of defects which act as charge trapsresulting in non-radiative transitions. Once heat-treated, however, thedefects were mostly eliminated, and increased crystallinity resulted inimproved emission. The 75 W sample (FIG. 18B) produced the highestemission intensity.

In summary, the as-deposited coatings were partially crystalline and didnot scintillate. However, thermal treatment of the coatings resulted inincreased crystallinity and fewer defects, leading to excellentscintillation properties.

Example 4 Chemical Vapor Deposition of Lu₂O₃:Eu Films

Many CVD processes use the metal chloride-H₂—CO₂ system [6-7]. In thisstudy, thermodynamic calculations using HSC chemistry simulationsoftware (see www.hsc-chemistry.net) were used to determine theviability of the CVD process. The hypothesized deposition reactionequation for Lu₂O₃:Eu³⁺ as shown in Eq. (1) was made using a combinationof Eqs. (2) and (3).(2−χ)LuCl₃(g)+(χ)EuCl₃(g)+3CO₂(g)+3H₂(g)=Lu_(2-χ)Eu_(χ)O₃(s)+3CO(g)+6HCl(g)  (1)2LuCl₃(g)+3CO₂(g)+3H₂(g)=Lu₂O₃(s)+3CO(g)+6HCl(g)  (2)ΔG _(rxn,2)=−439 kJ/mol of Lu₂O₃, 1000° C.2EuCl₃(g)+3CO₂(g)+3H₂(g)=Eu₂O₃(s)+3CO(g)+6HCl(g)  (3)The Gibbs free energy of reaction for Eq. (2) is −439 kJ/mol as opposedto a value of −170 kJ/mol for Eq. (3) at 1000° C. Although thisdifference in free energy could result in a variance between deposit andgas composition, it was favorable in this study, as low amounts of Euare desired in the coating deposits. Even though LuCl₃ and EuCl₃ aresolids at room temperature, their vapor pressure at depositiontemperatures (1000° C.) are high enough to provide an adequate reactantflow. Since the chlorides are extremely hygroscopic, they were generatedin situ by reacting lutetium and europium metal with judicious controlof the temperature and the chlorine flow rates. It is known that aeuropium concentration of 5-7 mol % in the Lu₂O₃:Eu³⁺ system yields thehighest emission intensity [8,9]. Furthermore, the ability to interpretan image is directly related to the emission intensity uniformity, andthus dopant uniformity is essential to the imaging process. Withknowledge of the variance in free energy of formation of Lu₂O₃ andEu₂O₃, the ratio of Lu and Eu in the internal chloride generator wasempirically determined in order to achieve the desired level of Eudoping. To maintain 5-7 mol % Eu in the deposit, both metals wereuniformly mixed to avoid excess preferential reactions. Europiumchloride melts above 730° C., and although lutetium chloride melts at925° C., it sublimates above 750° C. By combining elevated temperatureand low pressure, it is possible to ensure that the metal-chlorinereaction is the limiting kinetics and not the evaporation/sublimationrate, thus providing the necessary control.

The cold wall CVD reactor used an RF induction heater to heat thesubstrate and crucible using graphite susceptors (see FIG. 6). Thereactants used for deposition were the metal chlorides (LuCl₃ andEuCl₃), CO₂, H₂ and Ar as diluant. Excess H₂ was present to ensurecomplete reduction of metal chlorides. Process parameters ranged from950° C. to 1050° C. for both the substrate and the generator, between 50and 150 mbar, and a total flow rate of approximately 2 slm.Morphological analysis of the coatings was performed using a Zeiss VP40high resolution scanning electron microscope (SEM) in conjunction with aBruker D8 Focus X-ray diffractometer (XRD) in θ/2θ mode. Emissionproperties were confirmed using a Gatan MonoCL2 cathodoluminescencespectrometer.

Since many parameters affect the coating structure and properties,various configurations were designed and tested. One of the problemsencountered was the formation of metal oxy-chlorides due to a high metalchloride partial pressure and short mixing times. This was resolved bymaintaining a minimal distance (e.g., 60 mm or greater, depending on thereactor) between the substrate and the gas outlet to allow for propermixing and by supplying sufficient CO₂ and H₂ to fully reduce the metalchlorides.

Two of the key features in a CVD process are the fluid dynamics andtemperature profile of the gases as they approach the substrate. The gasvelocity and profile is determined by the total gas flow rate and theoutlet design. The temperature profile is determined by the crucible andsubstrate temperature, and the fluid dynamics. The way the gases passthrough the crucible and the flow rate are determining factors in theamount of thermal energy gained prior to mixing and determines thetemperature profile of the approaching gas. Modeling was performed usingthe Comsol multiphysics modeling software Femlab to obtain a basicinsight into the process. The simulated flow dynamics and temperatureprofiles are shown in FIG. 7A-D. This analysis is specific for the CVDreactor used and would have to be modified for a different reactor. Themodeling showed a clear relationship between the crucible temperature,substrate temperature, total flow rate, and temperature profile.

Further parameters were generally as follows. The amount of lutetiummetal was about 0.8 g and europium metal about 0.1 g. The vacuum was 75Ton. The flow rates were 6 sccm for Cl₂, 800 sccm for Ar, 312 sccm forCO₂, and 1250 sccm for H₂. Crucible temperature was 950° C. forsublimation of LuCl₃ and EuCl₃, and the substrate temperature was 1050°C.

This set-up configuration led to the deposition of coatings in acolumnar fashion with a strong orientation preference growth directlyfrom the first nucleated, equitaxial layer deposited on the substrate.Such microstructure is a result of high supersaturation and limitedlateral diffusion. This structure is desirable for radiation detectionsince every column would act as one ‘pixel’.

When grown at approximately 1000° C. on an amorphous quartz substratewith a growth rate of approximately 3.2 μm/hr, a columnar structureemerged as seen in FIGS. 9A and 11B. The columnar grains appear to havean average diameter of approximately 1.5 μm and a total coatingthickness of approximately 6.4 μm. When comparing the XRD plot in FIG.10A to a polycrystalline powder diffraction plot in FIG. 10B, a clear(100) orientation preference is visible. Such a preference for the (100)orientation indicates a free energy minimization for growth in thisdirection. Observations of the surface morphology in FIG. 9B revealedthe columnar growth to consist of stacked platelets or discs growingperpendicular to the (100) direction. Such a layer to layer formationhas been defined as Frank-van der Merwe (FM) growth and typically leadsto smooth surfaces.

Growth conditions were then modified by decreasing the ratio of metalchloride to unreacted chlorine while keeping the total chlorine flowrate constant. This led to the deposition of a highly facetted columnarstructure, as seen in FIG. 11, with a growth rate of 0.5 μm/hr and acoating thickness of approximately 2.4 μm. The columnar growth appearsto be single grained, with an average diameter of approximately 450 nmand with a clear surface morphology. This could be indicative of eithera cellular or dendritic growth or simply a surface effect. This type ofgrowth has been referred to Volmer-Weber (VW) and typically leads torough surfaces. The XRD plot in FIG. 12 combined with the SEM images inFIGS. 11A and 11B shows the preferred orientation to be in the (100)direction, perpendicular to the substrate surface. The ability todrastically tailor morphology, and size of the columnar grains via CVDprocessing parameters can be beneficially used to engineer coatings tofit specific applications.

Both lutetium and europium oxide have similar body centered cubic (BCC)lattice structure (Lu₂O₃=10.39 Å, Eu₂O₃=10.87 Å) and form a completesolid solution. For optimal emission to occur, europium must form asolid solution by substituting into the lutetium site of Lu₂O₃ as Eu³⁺.Although lutetium has only one stable oxidation state of +3, meaning itcan only exist as Lu₂O₃, europium can have either +2 or +3 as itsoxidation state, creating structures such as EuO, Eu₂O₃, and Eu₃O₄ orpotentially more. Thermodynamically, Eu₂O₃ is significantly morefavorable; however, it is possible to deposit non-equilibrium phases inCVD. Experimental results confirmed this possibility when solelydepositing europium yielded europium monoxide (EuO). It was hypothesizedthat as a result of the co-deposition, the europium would be forced intothe +3 valence. Furthermore, it is possible for europium oxide to form asecond phase rather than go into solution which would result innon-optimal emission. This was visible in certain circumstances where asecond phase of Eu₂O₃ was visible in the XRD plots, proving theformation of a solid solution to be difficult. However, XRD plot in FIG.12 shows no second phase, and the emission spectrum in FIG. 13 confirmedeuropium to be in the correct valence, proving that a solid solution hasbeen achieved. If Eu²⁺ were present, there would be a broad emissionpeak from 400 nm to 500 nm; however, only Eu³⁺ is visible, which hasmany peaks and the standard maximum intensity peak at 611 nm, whichcorresponds to the ⁵D₀-⁷F₂ transition.

Example 5 Effect of CO₂ Partial Pressure on CVD

A series of CVD experiments on amorphous quartz substrates, with acrucible-to-substrate separation of 1.25″ indicated that varyingdeposition conditions produce a wide range of coating morphologies, asshown in FIG. 8. One particularly sensitive parameter was the CO₂partial pressure, in which small changes drastically affected the growthmechanism. At low temperature and low CO₂ partial pressure, thedeposition did not show any evidence of a preferred orientation (FIG.8A). However, as the temperature was increased, the grain size increaseddue to increased diffusion and, counter-intuitively, the growth ratedecreased. In this case, no homogeneous deposition occurred, suggestingthat there is a decrease in the deposition driving force or a depletionof reactants prior to deposition. Upon increasing the CO₂ partialpressure, there was a sudden change in the growth mechanism, resultingin the formation of stacked platelets or discs growing preferentially inthe <100> direction, as shown in FIG. 8B. This was confirmed by XRDmeasurements, as shown in FIG. 10A.

Example 6 Emission Properties of Europium-Doped Lutetium Oxide Films

In contrast to the significant differences in morphology of Lu₂O₃:Eufilms as described above, their spectroscopic profile is relatively lesssensitive to the conditions of deposition. This is largely due to thefact that the emitted light is generated by optical transitions betweenstates of the 4f electronic shell, which is well shielded fromenvironmental influences by the surrounding 5d shell. The effect isexemplified in FIG. 14, where no perceptible difference in shape wasfound between the radioluminescence emission spectra of Lu₂O₃:Eu in theform of a CVD film (FIG. 14A) and a standard hot pressed ceramic (FIG.14B). The radiative decay time, at about 1 ms, was similarly unaffected.

While the spectral shape was not significantly altered by fabricationconditions, the emission intensity most decidedly was. This is becausethe excitation energy deposited into the host lattice by the ionizingradiation must travel a substantial distance through that lattice (asmobile electrons, holes, and excitons) before it can actually reach anemitting center. This process is quite vulnerable to the maligninfluence of lattice defects, which degrade both the speed andefficiency of the energy transport. It can be seen from FIG. 15 that asimple post-deposition annealing treatment had a profound effect on theefficiency of the scintillation from a deposited Lu₂O₃:Eu film, causinga monotonic increase in emission with annealing temperature. Theincrease in light emission for post-deposition heat treatment was abouttwo orders of magnitude for heat treatments over the range from 300° C.to 900° C.

Example 7 Imaging Properties of Europium-Doped Lutetium Oxide Films

In order to demonstrate the imaging performance of a Lu₂O₃:Eu filmaccording to the invention, the modulation transfer function (MTF) of agraphite-deposited PVD film was measured. The MTF is a measure of thecontrast in an image of black and white line pairs as a function oftheir spatial frequency, and provides a quantifiable value representingthe ability to distinguish small features as they become smaller andsmaller. In FIG. 16, the MTF curve for a CVD coating according to thepresent invention is compared with the MTF curve for a pixelatedceramic, as reported in the literature [5]. It is clear that the coatingof the present invention has an extremely good MTF curve, which ismarkedly superior to that of the conventional ceramic.

A radiographic (x-ray) image of an integrated circuit chip was obtainedusing a PVD scintillator coating according to the present inventionmounted on a CCD chip, in an arrangement as shown in FIG. 20. The imageso obtained is shown in FIG. 19. The image demonstrates very goodcontrast and dynamic range, and 25 μm bond wires in the integratedcircuit are clearly seen in the image. The image was acquired using amammography X-ray source operated at 28 kVp, 160 mAs, with the source tospecimen distance set at 66 cm.

While the present invention has been described in conjunction withcertain preferred embodiments, one of ordinary skill, after reading theforegoing specification, will be able to effect various changes,substitutions of equivalents, and other alterations to the compositionsand methods set forth herein. It is therefore intended that theprotection granted by Letters Patent hereon be limited only by thedefinitions contained in the appended claims and equivalents thereof.

As used herein, “consisting essentially of” does not exclude materialsor steps that do not materially affect the basic and novelcharacteristics of the claim. Any recitation herein of the term“comprising”, particularly in a description of components of acomposition or in a description of elements of a device, can beexchanged with “consisting essentially of” or “consisting of”.

REFERENCES

-   1. I. Shestakova, V. Gaysinskiy, J. Antal, L. Bobek and V. V.    Nagarkar, Nucl. Instr. and Meth. In Phys. Res. B 263, 234 (2007).-   2. E. Zych, J. Phys. Condens. Matter 14, 5637 (2002).-   3. Pierson, H. O. Handbook of Chemical Vapor Deposition. Noyes, Park    Ridge, N.J. (1992).-   4. Bunshah, R. F., et al., Deposition Technologies for Films and    Coatings, Development and Applications, Materials Sciences Series,    ed. Bunshah R. F., Noyes, Park Ridge, N.J. (1982).-   5. Farman, T. T., et al., Oral Surg., Oral Med., Oral Pathol., Oral    Radiol., & Endodontics 99:608-613 (2005).-   6. Pierson, H. O., Handbook of Chemical Vapor Deposition. Noyes,    Park Ridge, N.J. (1992).

7. Hitchman, M. L., Jensen, K. F., Eds., CVD Principles andApplications, San Diego Academic Press, London (1993).

-   8. Tojan-Piegza, J., et al., Comparison Of Spectroscopic Properties    Of Nanoparticulate Lu2O3:Eu Synthesized Using Different    Techniques, J. Alloy Compd. 308:123-9 (2008).-   9. Lempicki, A. et al., A New Lutetia-Based Ceramic Scintillator For    X-Ray Imaging, Nucl. Instrum. Meth. A 488:579-90 (2002).

What is claimed is:
 1. A method of preparing a radiological scintillatorcoating material by chemical vapor deposition (CVD), the methodcomprising the steps of: (a) providing LuCl₃, EuCl₃, CO₂, and H₂ asreactants and a substrate to be coated with the scintillator coatingmaterial; (b) reacting the reactants in a CVD reactor, whereby ascintillator coating comprising Lu₂O₃ and Eu₂O₃ is deposited onto thesubstrate.
 2. The method of claim 1, wherein the reactor is a cold wallreactor.
 3. The method of claim 1, wherein the reaction is carried outwith input gas ratios in the range of 0.2 to 2.0% Cl₂, 2.0 to 40% CO₂,10 o 30% Ar, with the balance being H₂.
 4. The method of claim 1,wherein the reaction is carried out at a substrate temperature in therange of about 900° C. to about 1400° C.
 5. The method of claim 1,wherein the LuCl₃ and EuCl₃ reactants are generated in the reactor byreacting Cl₂ gas with Lu metal and Eu metal.
 6. The method of claim 5,wherein the temperature and flow rates are regulated so as to avoid thebuildup of solid LuCl₃ on the Lu metal and Eu metal surfaces.
 7. Themethod of claim 1 resulting in a preferentially oriented columnar graingrowth of the scintillator coating.
 8. The method of claim 1, furthercomprising the step of: (c) annealing the scintillator coating by heattreatment at a temperature in the range of about 100 to 1400° C.